(1) Field of the Invention
This invention relates broadly to method and apparatus for providing a selected audience area with undistorted or intelligible sound.
More specifically, this invention relates to method and apparatus for providing each of a plurality of selected audience areas with undistorted or intelligible sound from one of a plurality of sound generating sources.
Even more specifically, this invention relates to method and apparatus for providing each of a plurality of selected audience areas, such as various locations in enclosed or unenclosed spaces, with undistorted or intelligible sound from one of a plurality of sound generating sources, such as multiple acoustical sources or loudspeakers mounted in close proximity to each other in a public address system, whereby the effects of intermodulation and other forms of interactive distortion among said plurality of sound generating sources are reduced.
(2) Description of the Prior Art
Various types of sound generating sources embodying acoustical speaker systems are known.
U.S. Pat. No. 2,955,669 (1960) to Rice discloses a triplanar hanging speaker enclosure utilizing two walls and a ceiling as sound baffles, wherein, it is said, the enclosure requires an absolute minimum of space and is highly efficient in speaker-to-air coupling. Rice neither discloses nor suggests reflecting indirect sound waves, generated as a necessary concomitant to the generation of direct sound waves, only to the audience area served by the speaker. More particularly, Rice neither discloses nor suggests the speaker enclosure serving each of a plurality of separate audience areas by means of one of a plurality of speakers, wherein indirect sound waves generated at one speaker are reflected only to the audience area served by the said one speaker and not the audience areas served by the other speakers. Rice does not teach how to reduce or eliminate the effects of intermodulation and other forms of interactive distortion among a plurality of speakers.
U.S. Pat. No. 2,544,742 (1951) to Volf discloses a loudspeaker unit containing a multiplicity of loud speakers directed in four or five different directions for use in an auditorium. It is said that this unit provides speeches or music clearly and without objectionable echoes. The unit is described as being exactly cubical and having four loud speakers mounted therein and facing in four directions 90.degree. apart, so that the sound is projected in all directions, and a fifth loud speaker centrally mounted in the bottom of the cabinet and facing downwardly. The cabinet is divided by partition into four compartments, one for each of the four radially mounted loudspeakers. Volf neither discloses nor suggests reflecting indirect sound, generated at one speaker as a necessary concomitant to the generation of direct sound by the said speaker, only to the audience area served by the said speaker. More particularly, Volf neither discloses nor suggests a speaker enclosure serving a plurality of separate audience areas by means of a plurality of speakers whereby indirect sound waves generated by one of the plurality of speakers is reflected only to the audience area served by the said one speaker and not to the audience areas served by the other speakers. Volf does not teach how to reduce or eliminate the effects of intermodulation and other forms of interactive distortion among a plurality of speakers. Quite to the contrary, Volf specifically teaches that the direct sound waves emanating from the five loudspeakers blend and coalesce in mid-air, at a distance equal to about twice the length of one edge of the cubical cabinet whereby, it is said, the smoothness and fidelity of the resonated sound waves are increased. Actually, the indirect sound waves in Volf's system generated at the speakers must also blend and coalesce, which is exactly contrary to the intent of the present invention.
The terms "direct sound" and "indirect sound," as used throughout the present specification and claims, can be most easily understood by considering a sound generating source or means in its most conventional form, viz. a loudspeaker having a conical diaphragm aimed at an audience area intended to be served by the loudspeaker.
As electrical currents in the coil of the loudspeaker fluctuate according to the electrical signals fed into the coil analogous to the frequencies of sound intended to be generated by the loudspeaker, the diaphragm will move at the desired frequency, causing positive and negative fluctuations in air pressure in front of it, which results in the generation of sound waves projected or directed toward the audience area. These sound waves are termed "direct sound" or "direct sound waves." The space in front of the diaphragm in which these positive and negative fluctuations in air pressure are generaated by the diaphragm may be termed the pressure zone of the diaphragm. The atmosphere adjacent this pressure zone reacts to the fluctuations of air pressure in the pressure zone with a finite delay to balance such positive and negative fluctuations, thus tending to restore the space around the diaphragm, including the pressure zone, to equilibrium which is ambient atmospheric pressure. This reaction of the atmosphere adjacent the pressure zone is a necessary concomitant to the generation of direct sound waves by the diaphragm and results in the generation of other sound waves which are termed "indirect sound" or "indirect sound waves" which are projected in a direction or directions generally away from the audience area.
If not reflected in the manner hereinafter described, such indirect sound waves could cause reverberation which, when reaching the audience area served by the loudspeaker, could so distort the direct sound waves projected to the audience area served by the loudspeaker as to render them unintelligible. It is these indirect sound waves which, when reaching audience areas served by other loudspeakers in a multi-speaker system, could result in intermodulation and other forms of interactive distortion of the direct sound waves generated by the other loudspeakers to such an extent as to render those direct sound waves unintelligible.
U.S. Pat. No. 3,964,571 (1976) to Snell discloses a loudspeaker system having acoustical reflector surfaces which minimize those reflections from an acoustic boundary which are out of phase with direct sound generated by a loudspeaker. Snell neither discloses nor suggests positioning an acoustical reflector relative to the loudspeaker in such manner that indirect sound is reflected only to the audience area served by the loudspeaker. Snell also discloses a multispeaker arrangement with acoustical boundaries as reflectors and with an acoustical absorber to minimize certain reflections which may sometimes be out of phase with direct sound, but does not teach reflecting indirect sound generated adjacent one loudspeaker only to the audience area served by the said loudspeaker and not to audience areas served by adjacent loudspeakers.
In addition to the sound generating systems disclosed in the prior art patents hereinbefore described in some detail, various other types of acoustical speaker systems are known. These are, for example, the systems illustrated in U.S. Pat. No. 3,912,866 (1975) to Fox, U.S. Pat. No. 4,280,586 (1981) to Petersen, and U.S. Pat. No. 4,473,721 (1984) to Klein. These are intended as an illustrative listing of U.S. prior art patents and not as an exhaustive listing of all prior art patents relating to acoustical speaker systems.